In an optical communication system, due to external factors applied to optical fibers and/or various optical elements, the polarization state of signal light is not always constant but changes with time.
Hence, in order to always obtain a constant optical gain by an SOA provided in the optical communication system, an optical gain generated in the SOA is required to be constant regardless of the polarization state of input signal light.
As a technique to realize a polarization independent SOA, for example, the following two techniques have been disclosed.
First, a technique (hereinafter referred to as “first technique”) has been disclosed in which in a semiconductor quantum well layer which is formed on an InP substrate and which is composed of a GaInAsP-based barrier layer and a GaInAsP-based well layer, the well layer receives no tensile stress in a in-plane direction but the barrier layer receives a tensile stress in the in-plane direction.
According to this first technique, when it is designed such that both the well layer and the barrier layer receive no tensile stress in the in-plane direction it is practically very difficult to make the amplification gain of the TM mode approximately equal to the amplification gain of the TE mode or larger than that by a certain value; hence, the structure described above has been used (see Japanese Laid-open Patent Publication No. hei4-27183).
In addition, in order to realize a polarization independent semiconductor optical amplifier in which the strain may not be increased larger than that of the first technique so as to obtain a desired effect, a technique has been disclosed (hereinafter referred to as “second technique”) in which a strained multi-quantum well active layer formed on a GaAs substrate and composed of GaAsP well layers and GaAsP barrier layers or a strained multi-quantum well active layer formed on a GaAs substrate and composed of InGaAs well layers and GaAs barrier layers is used, and in which an in-plane tensile stress is applied to both the well layers and the barrier layers (see Japanese Laid-open Patent Publication No. hei6-244508).
Incidentally, in recent years, in the optical communication system, a wavelength division multiplexing (WDM) transmission system has been used in many cases. In addition, as one type of WDM transmission system, a coarse WDM (CWDM) transmission system in which signal lights are disposed at relatively wide wavelength intervals (such as 20 nm) has been increasingly used.
In order to correspond to the transmission systems described above, it has been desired to realize an SOA having polarization independent optical amplification characteristics over an entire wide wavelength band that WDM and/or CWDM optical signals use.
In addition, the SOA generally has a wide optical amplification wavelength band (gain band) and, for example, can collectively amplify a plurality of CWDM signal lights; however, in order to make an optical output of each signal light constant, it is preferable to make the optical gain constant over a wide wavelength band. That is, it has been desired to realize an SOA having a flat optical gain spectrum in a wide band.
For example, as a technique to realize a polarization independent semiconductor optical amplifier over a wide wavelength band, there has been a technique (hereinafter referred to as “third technique”) capable of adjusting the difference in gain between TE polarization and TM polarization. According to this third technique, a plurality of InGaAsP active layers having different bandgap wavelengths are formed on an InP substrate, a wavelength band is increased by overlapping gain spectra of different wavelength bands obtained from the InGaAsP active layers, and a tensile strain is applied to each InGaAsP active layer while no strain is applied to InGaAsP barrier layers provided between the InGaAsP active layers (see Japanese Laid-open Patent Publication No. 2002-16322).
However, the above first and the second techniques are a technique to simply realize a polarization independent SOA, and it has not been taken into consideration to obtain polarization independent optical amplification characteristics over a wide wavelength band that, for example, WDM and/or CWDM optical signals use or over an entire region of a wide gain band (wavelength band) of the SOA, or it has not been taken into consideration to obtain a flat gain spectrum in a wide band.
In addition, although the third technique is a technique to realize a polarization independent semiconductor optical amplifier over a wide wavelength band, since the wavelength band is increased by overlapping gain spectra having different shapes, it has been difficult to obtain a flat gain spectrum in a wide band.